1. Field of the Invention
This invention relates to hoses having an inner tube, an outer cover and a reinforcement layer disposed therebetween, and more particularly to a power steering hose having enhanced heat-resisting properties.
2. Description of the Prior Art
Power steerings are a hydraulic power multiplier unit incorporated in steering mechanism of automotive vehicles. Hoses used in such unit are called "power steering hoses" and generally include an inner tube, a reinforcement layer of organic fibrous material such as nylon, and an outer cover laminated one on another.
The inner tubes of conventional power steering hoses are formed primarily of acrylonitrile butadiene copolymer rubber (NBR) which are prefereably for use in a temperature range of -40.degree.-+120.degree. C.
With an increasing tendency in the modern automobiles to be equipped with turbo-chargers and front-engine front-drive systems, there is a strong user's desire that power steering hoses be resistant to temperatures as high as 120.degree.-160.degree. C.
The conventional steering hoses having NBR inner tubes are however unsatisfactory in heat resisting properties and hence cannot withstand the desired high temperatures 120.degree.-160.degree. C.
With the foregoing difficulty in view, an attempt has been made to develop an inner tube formed by a polymer such as chlorosulfonated polyethylene rubber (SCM). The attempted development is however still short of the desired heat resistance.
According to another prior attempt, the inner tube of a power steering hose is composed of an inner layer formed of high-performance rubbers and an outer layer of conventional rubbers. A typical example of such double-layered inner tube has an inner layer formed of hydrogenated NBR and an outer layer of NBR. Hydrogenated NBR exhibits an improved resistance to heat and is known to be enhanced in its heat resistant properties by peroxide vulcanization.
Hydrogenated NBR, when used with NBR, however causes a problem that sulfur-vulcanized NBR fails to give sufficient adhesion to peroxide-vulcanized hydrogenated NBR while peroxide-vulcanized hydrogenated NBR is effective for adhesion to peroxide-vulcanized hydrogenated NBR, but not to reinforcement layers of organic fibrous materials. A further difficulty is that sulfur-vulcanized NBR and peroxide-vulcanized NBR when used concurrently often induce interfered reaction between sulfur and peroxide, failing to provide a stable adhesion or bondage therebetween and sometimes causing bulging or blister.
Other qualities are in many instances desired with respect to copper proofness, low-temperature flexibility, volumetric expansion, bursting strength and fitting mountability. However, they vary with the type of rubber compositions used. This problem precludes availability of a power steering hose capable of providing a good balance of all such qualities.